An extrasolar planet smaller than Mercury

Since the first exoplanet was found in 1995, researchers have progressively revealed that there are planetary systems very different to our own. First came the discovery of planets much larger, and even much hotter, than the members of the Solar System. Recently, and thanks to the extreme precision of Kepler space telescope, planets have been detected with sizes similar to that of Earth, or even smaller. Now, and for the first time, a planet smaller than Mercury has been found.

Kepler space telescope, launched in 2009, has the mission to determine, in our Galaxy, the abundance of rocky planets placed inside the habitable zone of stars similar to the Sun. Kepler constantly monitors some 150 000 stars in search for transits due to their planetary companions (transits are equivalent to eclipses in the Solar System).

Kepler got such transit signals during 978 days, indicating the existence of three planets, in the photometric time-series of a star similar to the Sun, although somewhat cooler, called Kepler-37 (also labelled as KIC 8478994 and KOI-245). It is estimated that the star size reaches 70 % of the Sun.

The planet just discovered, Kepler-37b, is the innermost in this system of three. According to David Barrado, Director of the German-Spanish Astronomical Centre (Calar Alto Observatory, Almeria, Spain), “Due to its extremely small size, similar to the Moon, and to its intensely irradiated surface, Kepler-37-b is most probably a rocky planet devoid of atmosphere or water, similar to Mercury.”

Techniques applied in other cases to confirm that changes in apparent brightness were due to planets orbiting the star, such as radial velocity variations or transiting time shifts, were of no use in this case. So, the scientists had to explore possible astrophysical scenarios (called false positives due to blending) that may emulate a planet transiting in front of Kepler-37. To this end, they used a specific and highly complex software called BLENDER.

Schematic diagram showing the configurations that may lead to false positives due to blending, in the search for planets by means of the transit method. (Click to enlarge.)

An additional observational technique was applied, with the instrument AstraLux attached to the 2.2 m telescope of Calar Alto Observatory, what provided extremely high resolution images of the star, with a quality similar to what may be reached with Hubble space telescope. As stated by Jorge Lillo-Box, scientist at CAB and who has also taken part in this work, “With the technique we have applied, called lucky imaging, we have been able to discard a good number of false positives due to blending or, in other words, we have eliminated configurations such as the presence of other stars or star spots. These phenomena may be confused with a planet because they induce similar effects on the data. For this reason, only an accurate observation allows to discard that the signal comes from stellar objects instead of planets.”

Part of the images were taken during the Spanish guaranteed time at the German-Spanish Astronomical Centre (Calar Alto Observatory, dependent from the Spanish Higher Council for Scientific Research CSIC and from its German equivalent, the Max Planck Society MPG), time managed by the Astrophysical Institute of Andalusia (IAA-CSIC). This shows the efficiency of dedicated programs, that make an intensive use of telescope time in projects trying to enlarge the frontiers of knowledge.

This work is fruit of a wide international collaboration devoted to exploit the extraordinary data provided by the Kepler satellite, but also data obtained from ground based telescopes. David Barrado underlines that “Without the additional observations made by ground based telescopes it would not have been possible to interpret correctly the information yielded by Kepler. The couple formed by space plus ground based observatory generates the synergies required to reach this kind of impressive discoveries, that some years ago were beyond reach of the wildest imagination. Again, this demonstrates the need to keep an adequate funding for ground based observatories.”

The case of Kepler-37b offers a clear example of the results that can be expected from telescopes of the two-meter class, such as the veteran Zeiss reflector of Calar Alto, when they are equipped with advanced technology instruments as AstraLux and they are made available to an internationally networked community that participates from the first line in Forefront science.

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